David Letterman, nearly a year and a half after leaving CBS' "Late Show," appears driven by a somewhat loftier goal: He wants to save the world.

Letterman guest hosts Sunday's installment of the National Geographic Channel’s "Years of Living Dangerously" ­– a gig that took him to India to report on climate change. In semi-retirement, as in his three-decade late night career, Letterman charts his own path, as he journeys toward perhaps his greatest human trick of all.

India, of course, is a long way from the New York TV studios where Letterman got legions of fans stuck on him with early stunts like donning a Velcro suit and leaping onto a Velcro wall. And the gravity of his latest venture marks a departure for the once perennially sarcastic host known for getting cursed out by one-named icons Madonna and Cher.

But Letterman not only grew on audiences, he grew up. His heart surgery in 2000 at age 52 brought out a more serious side. He provided unforgettable words of outrage and comfort after 9/11 ( “We're gonna try and feel our way through this, and we'll just see how it goes – take it a day at a time.”). He became a stronger interviewer, learning to listen and follow his curiosity, much like his hero, Johnny Carson.

Letterman never set out to be a hardcore political comedian, even if in later “Late Show” years, he highlighted President George W. Bush's battles with eloquence in "Great Moments in Presidential Speeches" segments. It's telling that Letterman's first major post-"Late Show" appearance was a surprise Donald Trump-themed “Top 10 List” delivered during a Steve Martin-Martin Short stage show.

Few, though, might have guessed Letterman would head to another continent to interview Indian Prime Minister Narendra Modi and energy experts. But the undertaking brims with the ambition and restlessness that propelled his comedy career. As he nears 70, the gray-bearded late night elder is cognizant not only of his entertainment legacy, but of his role as father to his 12-year-old son, Harry.

Other than storied lives in comedy, Letterman, Leno and Seinfeld a share a determination to stay on the public eye, on their own terms – unlike Carson, who all but disappeared after his 30-year run on "The Tonight Show." He might not be leaping back into comedy, but Letterman, in his own way, is proving himself a standup guy.